Prevent Ed25519 signature malleability

Add a check that ensures that the upper half s of an Ed25519 signature is
bounded by the group order, i.e, 0 <= s < order. This is required by the
Verify procedure in RFC 8032, section 5.1.7, step 1, and prevents simple
modifications of signatures such as adding (a multiple of) the group order
to the upper half of the signature.

Found with EdDSA testcase 63 of project Wycheproof.

ok beck jsing

1 files changed, 28 insertions, 1 deletions

diff --git a/src/lib/libcrypto/curve25519/curve25519.c b/src/lib/libcrypto/curve25519/curve25519.c
index c35863ef87..cd1b0c5802 100644
--- a/src/lib/libcrypto/curve25519/curve25519.c
+++ b/src/lib/libcrypto/curve25519/curve25519.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: curve25519.c,v 1.13 2022/11/09 17:45:55 jsing Exp $ */
+/*      $OpenBSD: curve25519.c,v 1.14 2022/11/17 19:01:59 tb Exp $ */
 /*
  * Copyright (c) 2015, Google Inc.
  *
@@ -24,6 +24,7 @@
  * The field functions are shared by Ed25519 and X25519 where possible.
  */
 
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -4671,10 +4672,22 @@ int ED25519_sign(uint8_t *out_sig, const uint8_t *message, size_t message_len,
   return 1;
 }
 
+/*
+ * Little endian representation of the order of edwards25519,
+ * see https://www.rfc-editor.org/rfc/rfc7748#section-4.1
+ */
+static const uint8_t order[] = {
+  0xed, 0xd3, 0xf5, 0x5c, 0x1a, 0x63, 0x12, 0x58,
+  0xd6, 0x9c, 0xf7, 0xa2, 0xde, 0xf9, 0xde, 0x14,
+  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10,
+};
+
 int ED25519_verify(const uint8_t *message, size_t message_len,
     const uint8_t signature[ED25519_SIGNATURE_LENGTH],
     const uint8_t public_key[ED25519_PUBLIC_KEY_LENGTH]) {
   ge_p3 A;
+  int i;
   if ((signature[63] & 224) != 0 ||
       x25519_ge_frombytes_vartime(&A, public_key) != 0) {
     return 0;
@@ -4690,6 +4703,20 @@ int ED25519_verify(const uint8_t *message, size_t message_len,
   uint8_t scopy[32];
   memcpy(scopy, signature + 32, 32);
 
+  /*
+   * https://tools.ietf.org/html/rfc8032#section-5.1.7 requires that scopy be
+   * in the range [0, order) to prevent signature malleability. This value is
+   * public, so there is no need to make this constant time.
+   */
+  for (i = 31; i >= 0; i--) {
+    if (scopy[i] > order[i])
+      return 0;
+    if (scopy[i] < order[i])
+      break;
+    if (i == 0)
+      return 0;
+  }
+
   SHA512_CTX hash_ctx;
   SHA512_Init(&hash_ctx);
   SHA512_Update(&hash_ctx, signature, 32);